Electrical power cables having a longitudinally folded, corrugated or smooth, metallic shielding tape with overlapping edge portions or abutting, or subtantially abutting, edge faces are well known in the art. See, for example, U.S. Pat. Nos. 3,651,244; 3,943,271 and 4,130,450. Such cables include a central stranded conductor with a semi-conducting shield therearound which is covered by a layer of insulation. Insulation shielding, in the form of a semi-conducting layer, is around the insulation, and a longitudinally folded, smooth or corrugated metallic tape is around the insulation shield. A protecting jacket is disposed around the metallic tape.
It is also known in the art that when the insulation of such cables is exposed to moisture, and in conjunction with high electrical stresses and high temperatures, "electrochemical trees" more commonly referred to as "water trees" are formed in the insulation which may result in premature cable failure.
It is known that the introduction of a sealant material between the strands of the conductor and between the insulation shield and the metallic shielding tape prevents or minimizes the longitudinal propagation or water within the cable structure. See said U.S. Pat. Nos. 3,943,271 and 4,130,450. However, it has been found that the mere introduction of sealant into such spaces is not entirely satisfactory when the sealant is merely asphalt/rubber or a polyester compound which is not water swellable.
For example, voids may be formed in the sealant during the application thereof or may be formed when the cable is punctured accidentally. Furthermore, the components of such a cable, being made of different materials, have different coefficients of expansion and the components are subjected to different or varying temperatures during manufacture, storage and/or operation of the cable which can cause the formation of voids.
In addition, when the edge portions of the metallic shielding tape overlap, there is a small space between the overlapping tape and the insulation shield adjacent to the edge of the underlying tape and there may be some spaces between the overlapping edge portions of the tape. If the tape is corrugated, there are spaces between the humps of the corrugations and the insulation shield. Such spaces may not be completely filled by the sealant when it is applied, but even if they are, voids can develop at such spaces when the cable, or its components, is subjected to temperature changes, expansion and bending.
Any such voids form locations for the retention of moisture which can cause the formation of the deleterious "electrochemical trees" in the cable insulation, and the conventional sealants used in the cables, being unaffected physically by water, cannot eliminate such voids.
Progress has been made to eliminate the longitudinal propagation of moisture problem by including a water swellable material in the sealant and at the overlapping portions of the metal shield strip. See, for example, U.S. Pat. Nos. 4,963,695 and 5,010,209. While such efforts have resulted in improved results, there still can be problems of moisture ingress at the overlapping portions of the metal shield strip due to the fact that in operation, the cable temperature can vary depending on the current carried by the cable conductor, e.g. from ambient temperature to a conductor temperature of 130.degree. C., which means that the components of the cable expand and contract. However, the expansion coefficients of the materials of adjacent cable layers can differ. For example, the volume expansion coefficient of insulating or semi-conducting materials can be thirty times the expansion coefficient of the metal usually used for the metal shield, e.g. copper or aluminum. Therefore, the layers expand at different rates, and if the metal shield is constricted, it can buckle and/or not return to its original size when cooled after heating, leaving voids which are deleterious to the electrical characteristics of the cable.
U.S. Pat. No. 3,943,271 suggests overcoming the possible rupture on the metal shield problem by not bonding the overlapping edge portions of the metal shield to each other and by flooding the interior of the cable with a sealant. However, such construction does not prevent moisture from entering into the interior of the metal shield because of gaps or channels produced between the overlapping edge portions with temperature cycling of the cable.
U.S. Pat. No. 4,145,567, naming two of the inventors named in U.S. Pat. No. 3,943,271, is stated to disclose an improvement over the construction shown in the latter patent, thereby recognizing that the construction disclosed in Pat. No. 3,943,271 does not provide a complete solution to the expansion and moisture ingress problems. In the cable construction described in Pat. No. 4,145,567, the overlapping edge portions are bonded together, such as by solder, welding, epoxy resin, etc., so that they cannot move with respect to each other, and the expansion problem is met by a cushioning layer between the cable core and the metal shield. However, the jacket adheres to the metal shield which either restricts expansion of the metal shield or the bond is ruptured with temperature cycling due to the expansion of the core. The patent also does not recognize problems with buckling of the metal shield when the overlapping edges of the metal strip cannot move with respect to each other.